Dissector tube



Jun17,1947. j c, LARSON 2,422,244

" DISSECTOR TUBE Filed Sept. 19, 1942 NVENTQ ture.

Patented June 17, 1947 DISSECTOR TUBE Chris C. Larson, Fort Wayne, Ind, assignor to Farnsworth Television and Radio Corporation, a corporation of Delaware Application September 19, 1942, Serial No. 458,929

3 Claims. 1

This invention relates generally to picture signal generating tubes and more particularly to a dissector tube wherein a picture signal is generated by deflecting an electron image with re spect to an apertured electrode.

There is a tendency in television dissector transmitting tubes for a small area at the center of the cathode to be adversely effected during op eration whereby in the received television image a black spot of appreciable area is caused to be reproduced at the center of the reproduced pic- It is believed that such black spots occur as a result of bombardment of the photoelectric cathode in the dissector tube by ions which destroy the photoelectric coating of the cathode in the vicinity of its central portion. In the process of manufacturing vacuum tubes, there is present in the finished tube minute quantities of various gases such as those present in the atmosphere and in the case of the dissector tube wherein there is a caesiated surface there is also present a caesium vapor pressure at normal temperatures.

Electrons emitted by the photoelectric cathode in response to a received light image, collide with atoms of caesium and possibly atoms ofother gases within the tube envelope and generate positive ions. Since these ions are repelled by the accelerating electrode coating within the tube and since they are negligibly affected by the focusing field, they are attracted to th center of the photoelectric cathode and bombard it, tending to destroy the photoelectric coating. After a few hours of service, tubes of the dissector type have been found to cause a black spot in the received image for the reasons outlined above.

The principal object of this invention is to provide in a television system a, method and apparatus for improving the quality of transmitted and received television images.

Another object of this invention is to provide a novel dissector tube including a means for preventing destruction of the photoelectric cathode therein.

A further object of this invention is to provide a dissector tube including an ion trap for collecting ions which may be generated within the tube envelope.

There is provided in accordance With this invention, a dissector tube consisting of an evacuated envelope, a photoelectric cathode at one end thereof, an accelerating electrode in the form of a metal coating on the inside of the envelope and a collecting electrode for collecting electrons emitted by the photoelectric cathode in response to received light. The tube is also provided with deflecting coils for deflecting an electron image and a focusing coil for focusing the electron image in the plane of the collecting electrode. There is also provided an ion trap which is in the form of a helix and consists of, for example, anickel Wire helix disposed adjacent the accelerating electrode and co-extensive therewith. Ions which may be generated within the tube are attracted to the helical ion trap instead of to the center of the photoelectric cathode, thereby preventing destruction thereof.

For a better understanding of the invention, together with other and further objectsthereof, reference is made to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

The single figure of the drawing is a plan view partly in section of a dissector tube constructed in accordance with this invention.

Referring to the drawing, there is provided a dissector tube consisting of an evacuated envelope I, a cathode element 2 having a conventional photoelectric coating deposited thereon, an accelerating electrode 3 and an electron collecting anode system 5 which includes electron multiplier elements as illustrated. There is also provided a focusing coil 1 for focusing electrons emitted from the photoelectric cathode 2 in the plane of the apertured anode 5. Deflecting coils 8 and 9 provide horizontal and vertical deflection of the electron image emitted from the photoelectric surface on cathode 2. For a more detailed description of the structural detailsand operation of the dissector tube, reference may be had to the U. S. patent of Bernard C. Gardner, No. 2,200,166 of .May 7, 1940.

Within the accelerating electrode 3 and disposed adjacent and co-extensive therewith, there is provided an ion trap consisting of an openwork or foraminous electrode as a helix III which may comprise a coil of nickel wire. Battery I2 is connected to the various electrodes in a conventional manner to energize them in a manner well known in the art. The helix I0 is connected directly to the photoelectric cathode 2 by conductor ll whereby it is at cathode potential. The electrical connections of the multiplier elements within system 5 are omitted for the sake of simplifying the drawing.

Since it is impossible to attain a complete vacuum, there must necessarily be certain gases within an evacuated envelope even though they exist in small quantities. Thus, there probably oxygen, inert gases, etc., and caesium gas. It is thought that atoms of caesium are most likely to be the primary cause of destruction of the central part of the photoelectric cathode coating exists in envelope l', sma11 quantitiesof nitrogen,

tioned heretofore, and generate positive ions.-

The positive ions because of their greater mass are negligibly affected by the magnetic focusing and scanning fields generated bythe focusing coil 1 and the scanning coils 8 and 9. Because of the geometrical relation between thecathode -2'and anode 3 there is formed an electrostatic field which causes the positive ions that are generated in the anode space to bombard the central portion of the photoelectric cathode 2 and-destroy it. The helix 10 serves as an ion trap in that it is at cathode potential and attracts positive ions to prevent them from ever reaching the cathode 2, thereby preventing destruction of the photoelectric surface.

It is not intended that this invention shall be limited to an ion trap in the form of a helix as any suitable means of attracting ions adjacent the accelerating electrode is within the scope of this invention. Furthermore, it will be obvious to those skilled in the art that helixiil may be connected to other elements of the tube and still serve its intended purpose. For instance, helix [9 will attract positive'ions when connected to an electrode having an intermediate potential between the cathode and anode potentials.

While there has been described what is at present considered the preferred embodiment of the invention, it will be obvious to those skilled in the art, that Various changes and modifications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What I claim is: 1. An image tube comprising an envelope hav- '4 ing at one end thereof a photoelectric cathode adapted to receive an optical image and at the other end thereof electron collecting means for receiving electrons from said cathode, an accelerating electrode surrounding the electron discharge path between said cathode and said electron collecting system, and an openwork ion collecting electrode substantially co-extensive and coaxial with and adjacent said accelerating electrode and connected to said cathode for collecting ions present in said discharge space.

'2. An image tube comprising an envelope having at one end thereof a photoelectric cathode adapted to-receive an optical image and at the other end thereof an electron collecting system for receiving electron from said cathode, an accelerating electrode surrounding the electron discharge path between said cathode and said electron collecting system, and an ion collecting electrode comprising a helical wire member co-extensiveand coaxial with andadjacent said accelerating electrode for collecting ions present in said discharge space.

3. An image tube comprising an envelope having at one end thereof a photoelectric cathode adapted to receive an optical image and at the other end thereof an electron collecting system for receiving electrons from said cathode, an accelerating electrode surrounding the electron dis:- charge path between said cathode and said electron collecting system, and an openwork ion collecting electrode co-extensive and coaxial with.

and adjacent said accelerating electrode for collecting ions present in said discharge space.

CHRIS C. LARSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENT Date Farnsworth Jan. 10, 1939 

